Method of manufacturing orthodontic appliance having reinforcing fiber preform

ABSTRACT

An orthodontic appliance such as a bracket is made of a polymeric material with reinforcing fiber structure embedded in the polymeric material. The reinforcing fiber structure includes relatively long filaments that extend about the perimeter of the bracket including the periphery of an archwire slot. In one embodiment, a method for making the bracket includes the steps of placing a tubular fiber preform in a mold assembly in an orientation aligned with a mesial-distal axis of the resultant bracket, and then closing the mold assembly in directions toward a central axis of the preform.

This is a division of application Ser. No. 07/903,568 filed Jun. 24,1992.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an orthodontic appliance and a method formanufacturing an orthodontic appliance that is made of polymericmaterial and reinforcing fiber embedded in the polymeric material.

2. Description of the Related Art

Orthodontic treatment Concerns movement of improperly aligned ormalpositioned teeth to desired orientations. Orthodontic treatment ofteninvolves the use of small appliances known as brackets that are securedto incisor, bicuspid and cuspid teeth, and buccal tubes that are securedto molar teeth. The brackets and buccal tubes have small slots orpassages that receive a resilient, metallic archwire. The archwire formsa track to guide movement of the teeth, and the teeth are urged towarddesired positions by bends or twists placed in the archwire, or by theuse of other devices such as elastomeric modules.

Many brackets and buccal tubes have been made of stainless steel becausestainless steel is strong, non-absorbent, weldable and relatively easyto form and machine. However, adults and older children undergoingorthodontic treatment are sometimes embarrassed by the "metallic smile"appearance of such brackets. Further, certain patients are allergic tonickel and chromium that are often present in stainless steelappliances.

Orthodontic appliances are sometimes made of materials other than alloyscontaining nickel and chromium and can be used for patients sensitive tothese elements. For example, orthodontic appliances may be made of aceramic such as monocrystalline or polycrystalline alumina. U.S. Pat.No. 4,954,080 describes orthodontic appliances made of a polycrystallineceramic having a translucency which minimizes visibility of theappliance when mounted on a tooth so that the problem of a "metallicsmile" is largely avoided.

However, ceramic brackets are known to present a greater resistance tomovement of the bracket relative to the archwire as compared to metalbrackets. Resistance to movement is considered a disadvantage becausethe resistance slows movement of the teeth to positions desired by theorthodontist and can lengthen treatment time.

Orthodontic appliances have also been made of a plastic material that insome instances is translucent and neutral in color. Preferably, theselected plastic material is relatively resistant to staining by foodand beverages such as mustard, spaghetti sauce and grape juice. It isalso desired that the plastic material be relatively resistant to creepso that the sides of the archwire slot do not unduly deform.

Attempts have also been made to strengthen plastic orthodonticappliances by use of reinforcing fibers or whiskers. For example,plastic brackets containing fibers were promoted by AmericanOrthodontics of Sheboygan, Wis. as early as 1985. U.S. Pat. No.5,078,596 discloses an orthodontic bracket made of polycarbonate andtranslucent glass or ceramic fibers. It has been suggested that bracketsmay be made by injecting both resin and chopped fibers into a moldassembly.

U.S. Pat. No. 4,717,341 describes an orthodontic bracket produced fromglass-filled polycarbonate composite material by injection into a heatedaluminum mold. An opening to the mold is placed in the base directlyunder the center of the slot apparently in an attempt to achieve acertain orientation of the fibers.

There is a continuing interest in making orthodontic appliances as smallas practicable so that for aesthetic reasons the visibility of theappliance is reduced. However, it is important that the appliance havesufficient strength to resist breakage during ordinary use. Orthodonticappliances are often subject to significant stress during masticationwhen the bracket may contact hard food portions, occluding teeth orappliances mounted on occluding teeth.

In addition, certain smaller sections of appliances need to withstandrelatively strong forces that may be intentionally induced duringorthodontic treatment. For example, brackets often have tiny tiewingsshaped like hooks on opposite sides of the archwire slot for receiving awire or elastic ligature that is used to bind the archwire to thebracket. Such tiewings, if weak, may break apart from the body of thebracket when an attempt is made to firmly seat the archwire in thebottom of the slot. As can be understood, replacement of fracturedbrackets is a nuisance for both the orthodontist and patient.

There is a continuing need in the art for a plastic orthodonticappliance that is relatively small and yet has sufficient strength andstiffness, particularly in regions of the tiewings and sections near thearchwire slot. Preferably, such a bracket would be made according to amethod that is relatively inexpensive, adaptable for automation and yetprovides consistent, satisfactory results.

SUMMARY OF THE INVENTION

The present invention is directed toward an orthodontic appliance thatcomprises a body having an elongated archwire slot and a mesial-distalreference axis. The archwire slot has a certain periphery in referenceplanes generally perpendicular to the mesial-distal reference axis. Thebody is made of a polymeric material and reinforcing fiber structureembedded in the polymeric material. The reinforcing fiber structureincludes at least one filament and comprises at least one layer. The atleast one layer includes a number of aligned, side-by-side stretches ofthe at least one filament that lie in the reference planes and extendabout a majority of the periphery of the archwire slot.

The orthodontic appliance in accordance with the invention hasrelatively long fibers that provide the appliance with significantstrength in narrow sections that may be subject to relatively highstress. For example, an orthodontic bracket of the invention hassignificant stiffness and resistance to fracture in narrow sections ofthe bracket between the archwire slot and the tiewing undercuts and alsoin sections between the bottom of the slot and the base. Alignment ofadjacent filament stretches in an orientation perpendicular to themesial-distal reference axis enables the bracket body to satisfactorilywithstand torquing forces that may be imposed by an archwire in theslot. The long, oriented fibers enable the appliance to be made withrelatively high ratios of fiber to resin and selected anisotropiccharacteristics so that the resultant appliance exhibits satisfactorymodulus of elasticity, sheer strength and creep properties in comparisonto appliances made with shorter fibers.

The invention also concerns a method for making an orthodonticappliance. The method includes the steps of forming a fiber preformhaving a central axis. A quantity of polymeric material and the fiberpreform are placed in a cavity of a mold assembly in a position whereinthe central axis of the preform is generally parallel to an axis of themold cavity that corresponds to a mesial-distal axis of the resultantappliance. The mold assembly is closed in directions generally towardthe central axis of the preform.

A method for making an orthodontic appliance according to anotherembodiment of the invention comprises the steps of forming a fiberpreform having a central axis with an overall length at least generallyequivalent to the mesial-distal length of the resultant appliance. Aquantity of polymeric material and the fiber preform are placed in acavity of a mold assembly in a position wherein the central axis of thepreform is generally parallel to an axis of the cavity that correspondsto a mesial-distal axis of the resultant appliance. At least onefilament of the preform is caused to move along a reference planegenerally perpendicular to the central axis and toward an orientation atleast partially about a lateral periphery of an archwire slot of theappliance as the mold assembly is closed.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of a preform and mold assembly formaking an orthodontic appliance in accordance with the invention,showing the mold assembly in an initial orientation;

FIG. 2 is a view somewhat similar to FIG. 1 except that the moldassembly is shown in a closed orientation to form the appliance (that isshown in section along lines 6--6 of FIG. 8);

FIG. 3 is a view somewhat similar to FIG. 2 except that the moldassembly is shown in a partially retracted orientation;

FIG. 4 is a view somewhat similar to FIG. 3 except that the moldassembly is shown in a fully retracted orientation;

FIG. 5 is an enlarged, fragmentary, side cross-sectional view of thepreform and mold assembly shown in FIG. 1;

FIG. 6 is an enlarged, fragmentary, side cross-sectional view of themold assembly and appliance shown in FIG. 2, wherein the appliance istaken along lines 6--6 of FIG. 8 and cross-hatching of the appliance hasbeen deleted for clarity;

FIG. 7 is an enlarged perspective view of the preform shown in FIG. 5;

FIG. 8 is an enlarged perspective view of an appliance made with thepreform of FIG. 7;

FIG. 9 is an enlarged perspective view of a preform in accordance withanother embodiment of the invention;

FIG. 10 is a schematic view somewhat similar to FIG. 5 but showing apreform in accordance with yet another embodiment of the invention; and

FIG. 11 is a schematic view somewhat similar to FIG. 5 but showing apreform in accordance with a further embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An orthodontic appliance 20 according to one embodiment of the inventionis a bracket as shown in FIG. 8 and includes a tooth-facing base 22having a compound contour to match the contour of the underlying toothsurface. The base 22 is connected to a central body 24 that, in turn, isconnected to an occlusal tiewing 26 and a gingival tiewing 28. Thetiewings 26, 28 have mesial and distal portions partially separated by anotch 29. Alternatively, mesial and distal portions could be fullyseparated from one another as in the case of a "twin" bracket.

The tiewings 26, 28 are spaced from each other by an elongated archwireslot 30 having a central portion 31 with a rectangular, generallyU-shaped periphery when observed in directions perpendicular to amesial-distal reference axis of the appliance (that, for some brackets,is co-linear with the longitudinal axis of the slot 30). The centralportion 31 of the slot 30 has a width and depth closely complemental tothe dimensions of an archwire having a rectangular cross-sectionalconfiguration. An occlusal tiewing undercut 32 lies between the base 22and an occlusal edge of the mesial and distal portions of the occlusaltiewing 26, and a gingival tiewing undercut 34 lies between the base 22and a gingival edge of the mesial and distal portions of the gingivaltiewing 28. The undercuts 32, 34 are in the nature of elongated grooves.

The appliance 20 has a mesial side 36 and a distal side that is notshown in the drawings but is on a side of the appliance opposite themesial side 36. The undercuts 32, 34 and the archwire slot 30 extendacross the appliance 20 from the mesial side 36 to the opposite, distalside in a direction parallel to the mesial-distal axis of the appliance20. As an alternative, the archwire slot of brackets for certain teethis inclined in occlusal-gingival directions relative to both themesial-distal axis and the tiewing undercuts to provide an orthodonticpreadjustment known as angulation.

The appliance 20 is made of polymeric material and a length ofreinforcing fiber structure embedded in the polymeric material. Thereinforcing fiber structure has at least one layer that includes anumber of side-by-side stretches of one or more filaments that extend inaligned relation in reference planes generally perpendicular to themesial-distal axis of the appliance 20. Preferably, the alignedstretches extend about a majority of the extent of the U-shapedcross-sectional periphery of the archwire slot 30, and more preferablyabout the entire extent of the perimeter of the body 24 in suchreference planes including the slot periphery, so that continuousstretches of the filaments extend across sections of the appliance 20that are typically relatively narrow.

One satisfactory reinforcing fiber structure is a preform 38 as shown inFIG. 7 that is initially hollow and cylindrical. The tubular preform 38is made by winding a multi-strand fiber or tow 39 having a number oflong, continuous filaments 40 about the central axis of a bobbin in ahelical, tightly coiled, multiple-layer fashion with adjacent stretchesof the tow 39 in the same radial plane or layer being in side-to-sidecontact with one another. As an alternative, adjacent stretches of thetow 39 in the same radial plane could lie spaced apart from one another.Preferably, the layers lie in face-to-face, tightly overlapped relationto one another. Optionally, some portions of the tow 39 extend in adirection parallel or generally parallel to the central axis of thepreform 38.

As another alternative, the preform may be a woven fabric havingfilaments extending in cross directions. The fabric may initially have amulti-layered coiled configuration or a single layer, U-shapedconfiguration such that some of the filaments in the resultant applianceextend in planes perpendicular to its mesial-distal axis, while otherfilaments extend in directions parallel to its mesial-distal axis.

The filaments 40 are preferably made of a transparent or translucentmaterial such as glass, ceramic or quartz. Preferably, the polymericmaterial is also transparent or translucent, and the index of refractionof the filaments 40 is approximately equal to the index of refraction ofthe polymeric material so that light entering the labial surface of theappliance 20 is transmitted through the appliance 20, reflected off ofthe surface of the tooth and then retransmitted through the appliance 20toward its labial surface. In this manner, the appliance 20 takes on thecolor of the underlying tooth and blends with adjacent tooth structure.

The polymeric material is a thermoset or thermoplastic material that hassufficient strength to resist undue creep, deformation or fracture.Preferably, the polymeric material is relatively resistant to stainingby food and beverages such as mustard, spaghetti sauce and grape juice.

Suitable thermoset resins include epoxies (such as TACTIX brand epoxy(no. 123/H41, Dow Chemical) and EPON brand resin (Shell Chemical)),acrylics, polyesters, polyurethanes and mixtures thereof. Suitableacrylics include diglycidyl-methacrylate of hisphenol A ("Bis-GMA"),triethyleneglycol dimethacrylate ("TEGDMA"), polyethyleneglycoldimethacrylate 600 ("PEGDMA 600"), urethane dimethacrylate, andtrimethylolpropane triacrylate. A presently preferred thermosettingresin is a 30:36:33 mixture by weight of Bis-GMA, TEGDMA and PEGDMA 600respectively.

Suitable thermoplastic resins include acrylics (such as nos. NAS 30, NAS50 or ZYLAR 90, Novacor, or no. XT250, CYRO Industries). Other suitablethermoplastic resins include polysulfones (such as UDEL brandpolysulfone, Amoco), polycarbonates (such as LEXAN brand polycarbonate,GE) polyesters or polyurethanes. A presently preferred thermoplasticresin is NAS 50.

Preferably, the filaments 40 are fully wetted by the polymeric materialbefore the latter is shaped (and cured, if the material is athermosetting material) in order to enhance the strength andtranslucency of the resulting appliance 20. The filaments 40 arepreferably fully wetted before winding into the shape of the preform 38.In general, if the polymeric material is a liquid, the filaments 40 arewetted by dipping the tow 39 in a tray of the liquid. Once shaped to acylinder, the wetted preform 38 is fixated by partial curing or byfreezing to increase the viscosity of the polymeric material andfacilitate handling of the preform 38. If the polymeric material is athermoplastic, the tow 39 is electrostatically coated with thethermoplastic powder and then heated in an oven to melt the powder.Final shaping (and curing, if the material is a thermosetting material)of the impregnated preform 38 occurs during molding.

A mold assembly 42 is shown in FIGS. 1-6 and includes a plunger 44 and aclamp 46, both of which are movable in vertical directions viewing thedrawings. Although not shown, the plunger 44 and the clamp 46 are eachconnected to a respective double-acting hydraulic piston and cylinderassembly. A controller that includes a mini-computer is connected toeach piston and cylinder assembly in order to enable independent, timedmovement of the plunger 44 and the clamp 46 in the sequence describedbelow.

As shown in more detail in FIGS. 5 and 6, a central portion of the clamp46 includes an upstanding projection 48 that is of a size andorientation to form the archwire slot 30 in the resultant appliance 20.Sections of the clamp 46 adjacent the projection 48 are concave and formsmoothly curved labial surfaces of the tiewings 26, 28.

The plunger 44 includes a central, depending section having a lowermostconvex surface with a compound contour in order to form a contour on thebase 22 that matches the curvature of the tooth. The lowermost surfaceof the plunger 44 is preferably scored, knurled or grooved so that theresultant base 22 has surface structure for controlling or facilitatingmovement of the adhesive used to bond the appliance 20 to a tooth as theappliance 20 is pressed toward the surface of the tooth.

The mold assembly 42 also includes four slides 50 for molding theocclusal, mesial, distal and gingival sides of the appliance 20. The twoslides 50 that are shown in the drawings form the occlusal and gingivalsides, and each include a protrusion 52 for shaping the tiewingundercuts 32, 34 respectively as illustrated in more detail in FIGS. 5and 6. Surfaces of the slides 50 next to the protrusions 52 are curvedto provide a smooth contour for remaining occlusal and gingival surfacesof the appliance 20 from the base 22 to the labial surfaces of thetiewings 26, 28.

An upwardly-facing, outer inclined surface 54 of each of the four slides50 is in sliding contact with an inner, downwardly-facing inclinedsurface of a clamp ring 56 that is secured in a cavity of a load frame58. An outer, inclined, downwardly-facing surface 60 of each slide 50 isin sliding contact with an upwardly-facing, inclined surface of a camring 62 that is also secured within the cavity of the load frame 58. Aspacer 64 lies between the cam ring 62 and the clamp ring 56.

An inner, downwardly-facing, inclined surface 66 of each slide 50 isslidingly engageable with an upwardly-facing, inclined surface of theclamp Additionally, an inner, upwardly-facing, inclined surface 68 ofeach slide 50 is slidingly engageable with an inclined,downwardly-facing surface of the plunger 44. The two slides 50 shown inthe drawings are movable in a generally horizontal direction in theplane of the drawings, while the two remaining slides 50 not shown inthe drawings are movable in a generally horizontal directionperpendicular to the plane of the drawings.

In operation, the preform 38 is introduced into the mold cavity of themold assembly 42 in the orientation depicted in FIGS. 1 and 5 whereinthe preform 38 lies atop the projection 48 and substantially parallelwith the longitudinal axis of the protrusions 52 for forming the tiewingundercuts 32, 34. The preform 38 is placed in the mold cavity in aposition wherein the central axis of the preform 38 extends in adirection that corresponds to an axis extending between mesial anddistal sides of the resultant appliance. The preform 38 is introducedinto the mold cavity through a feed tube that leads from a bobbin winderassembly for winding the tow 39 into the preform 38. Preferably, thebobbin winder assembly is associated with several feed tubes that directthe preforms to a number of mold assemblies in turn.

Next, the clamp 46 is held in a stationary position while the plunger 44is moved downwardly toward the clamp 46 to flatten the preform 38against the projection 48 and cause occlusal and gingival portions ofthe preform 38 to drape over the projection 48. Subsequently, theplunger 44 is lifted while the clamp 46 is raised. As the clamp 46ascends, sliding contact of surfaces 66 of the slides 50 against theclamp 46 and sliding contact of the surfaces 54 against the clamp ring56 urge the slides 50 to move into the mold cavity toward one anotherand toward the preform 38. The clamp 46 is raised a distance sufficientto move the four slides 50 inwardly to desired positions for molding themesial, distal, occlusal and gingival sides of the resultant appliance20 including the tiewing undercuts 32, 34.

Next, the plunger 44 is lowered while the clamp 46 is held in astationary orientation to close the mold cavity of the mold assembly 42as shown in FIG. 2 and to press the impregnated preform 38 to its finalform matching the configuration of the desired appliance 20. To cure thepolymeric material, heat is introduced into the mold cavity by electricresistance heaters embedded in the clamp 46 near the projection 48 or byan induction coil surrounding the clamp 46. If the material isthermoplastic, the mold assembly is heated before closing and cooledbefore opening; for a thermoset material, the assembly is closed beforeheating.

As the mold assembly 42 is closed, the shape of the preform 38 ischanged from its initially cylindrical shape (FIG. 5) to the shape shownin FIG. 6 where the tow 39 including the filaments 40 extends around theperimeter of the appliance 20 in sections taken perpendicular to themesial-distal axis of the appliance 20. More particularly, closing ofthe mold assembly 42 causes the tow 39 to be draped over the projection48 such that the tow 39 ultimately extends around the archwire slot 30in a direction transverse to the length of the archwire slot 30, alongthe labial portions of the tiewings 26, 28, around the curved surfacesof the tiewing undercuts 32, 34 and along the curved surface of the base22. The resultant configuration of the preform 38 causes the density ofthe filament 40 (i.e., the weight ratio of the filaments 40 to thepolymeric material) to be greatest in narrow sections of the bracket,namely in the relatively thin sections between the archwire slot 30 andthe undercuts 32, 34, and in the section between the archwire slot 30and the base 22.

Preferably, the outer circumference of the impregnated preform 38 isapproximately equal to the perimeter of the appliance 20 (including allsurfaces of the archwire slot 30 and the undercuts 32, 34 facing theatmosphere) in sections transverse to the mesial-distal axis of theappliance 20 so that when the preform 38 is collapsed and compressed inthe mold assembly 42 the outer windings of the tow 39 are closelyadjacent external surfaces of the appliance 20. Additionally, the innerdiameter of the impregnated preform 38 is selected such that the initialvolume of the impregnated preform 38, ignoring the volume of its hollowinterior, is only slightly greater than the volume of the resultantappliance 20. As a result, when the mold assembly 42 is closed, theappliance 20 is formed without voids or spaces, and yet withoutexcessive amounts of polymer or fiber material that might otherwiseunduly hinder closure of the mold assembly 42. As an example, a hollowpreform for making a bracket somewhat similar to appliance 20 but havinga uniform cross-section across its mesial-distal extent identical to thecross-section shown in FIG. 6 (i.e., lacking notches 29 and cut-awayarchwire relief areas next to its mesial and distal sides) has an outerdiameter of 3.9 mm, an inner diameter of 3.2 mm and an overall length of3.6 mm.

When the mold assembly 42 is in its closed position as shown in FIGS. 2and 6, excess polymer material is urged upwardly along small externalchannels formed in the plunger 44 to a small undercut region that servesas an overflow reservoir. As the polymer material in the mold cavity iscured, the polymer material in the channels and the overflow reservoiris also cured and thereafter functions as a handle for retaining themolded appliance 20 on the plunger 44 during opening of the moldassembly 42.

An initial stage of opening the mold assembly 42 is shown in FIG. 3 andis carried out by shifting the plunger 44 downwardly while releasingpressure in the hydraulic cylinder connected to the clamp 46 so that theclamp 46 is no longer stationary and is somewhat free to descend. As theplunger 44 is urged downwardly, the plunger 44 slides against thesurfaces 68 of the slides 50, urging the surfaces 60 to slide againstthe cam ring 62 and thereby cause the slides 50 to move outwardly andaway from the mold cavity until the protrusions 52 clear respectiveocclusal and gingival sides of the appliance 20.

Next, the piston and cylinder assemblies are pressurized to move theplunger 44 upwardly and the clamp 46 downwardly until the mold assembly42 is in its retracted or open orientation as illustrated in FIG. 4. Theplunger 44 is then raised a distance sufficient to position theappliance 20 above the load frame 58.

Next, a trough (not shown) is moved to a location beneath the appliance20 and a pin (also not shown) is moved to eject the appliance 20 fromthe bottom of the plunger 44. The pin is located within the central,lower portion of the plunger 44 and, when moved toward a lowermost,extended position, is operable to sever the appliance 20 from the curedexcess polymer material forced from the mold cavity and into thechannels and overflow reservoir. The above sequence of operation of themold assembly 42 is then repeated to make additional appliances asdesired.

A preform 138 in accordance with another embodiment of the invention isshown in FIG. 9 and comprises a central core of a wound tow 139 thatextends substantially along the entire mesial-distal extent of theresultant appliance. Additional windings of the tow 139 are provided atmesial and distal sides of the central core as shown at 141. Theadditional windings at the locations 141 facilitate complete filling ofthe mold cavity for appliances having additional volume in mesial anddistal regions (e.g., in instances such as "twin" brackets having mesialand distal tiewings completely spaced apart from each other, whereadditional resin and fiber are desired on mesial and distal regions).

A preferred preform 238 is illustrated in FIG. 10 and has a generallytrapezoidal cross-sectional configuration in sections perpendicular toits mesial-distal axis. The trapezoidal shape is advantageous forfacilitating lateral movement of the preform 238 to the desiredcross-sectional configuration of a bracket as the mold assembly 42 isclosed. Viewing FIG. 10, the lower, outermost corners of the preform 238provide enhanced flow of the tow 239 and polymer material into tiewingportions of the molded appliance.

An alternative preform 338 is shown in FIG. 11. The preform 338 is madeby altering the trapezoidal configuration of the preform 238 shown inFIG. 10 by placing the preform 238 in a mechanism (not shown) havingarms that form indentations 337 into sides corresponding to occlusal,gingival and labial surfaces of the resultant appliance. The preform 338is partially fixated in the mechanism in order to facilitate subsequenthandling so that the preform 338 will substantially retain its shapeuntil in the mold cavity.

The following paragraphs describing preparation of test articles andalso an example according to the invention should not be construed aslimiting the scope of the invention. Unless otherwise indicated, allparts and percentages are on a weight basis.

PREPARATION OF TEST ARTICLES

A thermosetting resin mixture was prepared by adding one part dicumylperoxide catalyst (Luperox 500R, ATOCHEM) to 100 parts of a 30:36:33mixture of BisGMA, TEGDMA and PEGDMA 600 . (no. 252, Sartomer)

Glass fiber tow (no. S-2 449-1250, Dow Corning) was cut into 10 cmsegments and the segments were laid in a side-by-side single layerarrangement in a tray containing the resin mixture. The tray was part ofa stop mold assembly having a rectangular cavity of 10 cm×10 cm and wascoated with a fluorocarbon mold release agent prior to use. She resinmixture was added to the tray, and the tow segments were allowed to soakfor 90 minutes in the resin mixture while degassed in a bell jar.

The mold was partially closed and excess resin was squeezed from thetray by gravity pressure of the top mold segment. The partially closedmold assembly was further degassed in a bell jar for 30 minutes, andthen placed in a press with sufficient pressure to complete closure ofthe mold. The mold was heated by press platens at 90° C. for 16 hours,then at 120° C. for 2 hours. The heat source was then deactivated andthe mold allowed to cool to a temperature below 7.0° C. and then opened.

The cured fiber reinforced material was visually observed under amicroscope and found to be translucent. Data from physical testing ofthe material showed a flexural yield strength of 370±10 MPa and aflexural modulus of 12410±660 MPa, satisfactory for orthodonticappliances.

Other test articles were made using a thermoplastic acrylic resin (NAS50, Novacor) that was ground into a powder having an average particlesize of 70 microns. The resin powder was coated on a glass fiber tow(no. S-2 449-1250, Dow Corning) by Electrostatic Technology, Inc. usingpowder coating technology such that the resultant tow had a resin tofiber ratio of 70:30.

The coated glass fiber tow was cut into 10 cm segments and the segmentswere laid in a side-by-side arrangement in multiple layers in the trayof the rectangular cavity stop mold assembly described above. The moldassembly was partially closed and placed between two press platens thathad been preheated to 200° C. The platens were moved to contact the moldassembly and the temperature-was monitored by a thermocouple attached tothe mold assembly. The mold assembly reached 175° C. in about 15minutes. At this time, the platens were pressed against the moldassembly with sufficient force to obtain closure of the mold, and theheat source was then deactivated. Once the mold assembly cooled to atemperature below 70° C., the assembly was opened.

The molded sheet was observed to be translucent. Data from physicaltesting of the sheet showed that the material exhibited a flexural yieldstrength of 480±20 MPa and a flexural modulus of 15170±280 MPa,satisfactory for orthodontic appliances.

EXAMPLE

A 10 cm length of the coated glass fiber tow mentioned above having athermoplastic resin to fiber ratio of 70:30 was tightly wound around amandrel that had been previously coated with a fluorocarbon releaseagent. A blower having a heater was directed at the tow during winding,and was subsequently deactivated to enable the wound preform to cool andretain its shape.

A mold assembly was provided for making an elongated integral bar ofmultiple bracket bodies that could be cut apart to produce individualbrackets. The mold assembly included a stationary bottom wall formolding a base of the bracket bodies, and a top, vertically movableplunger for forming an archwire slot. Two slides, movable along the basein a generally horizontal direction, provided tiewing undercuts whenurged in a direction toward the preform.

The impregnated, shaped preform was removed from the mandrel. The coiledpreform was then cut to a length of about 2.5 cm and placed in the moldassembly with its longitudinal axis in a direction parallel with theoverlying mold structure for forming the archwire slot and aligned withelongated protrusions on the slides for forming the tiewing undercuts.The mold assembly was partially closed, placed in the press having itsplatens preheated to 175° C. and the platens were moved to contact themold assembly. The mold assembly reached 175° C. in 10 minutes, afterwhich the platens were moved to fully close the mold. The heat sourcewas then deactivated, the platen allowed to cool to 100° C. and the moldassembly was opened.

The molded bar was visually inspected under a microscope, and it wasobserved that the glass fibers extended along a path next tosubstantially the entire perimeter of the bar, including surfacesrepresenting the base, tiewings and tiewing undercuts. The bar was alsovisually observed to be translucent.

We claim:
 1. A method for making an orthodontic bracket comprising thesteps of:forming a fiber preform having a central axis with an overalllength at least generally equivalent to the mesial-distal length of theresultant bracket; placing a quantity of polymeric material and thefiber preform in a cavity of a mold assembly in a position wherein thecentral axis of the preform is parallel to an axis of the mold cavitythat corresponds to an axis extending between mesial and distal sides ofthe resultant bracket; causing at least one filament of the preform tomove along a reference plane generally perpendicular to the central axisand to an orientation at least partially about a lateral periphery of anarchwire slot of the bracket as the mold assembly is closed; and curingthe polymeric material to make the bracket.
 2. The method of claim 1,wherein said step of forming said fiber preform includes the step ofwinding at least one filament around a bobbin.
 3. The method of claim 2,wherein said step of winding at least one filament around a bobbinincludes the step of winding a fiber tow having a number of filamentsabout a bobbin.
 4. The method of claim 1, wherein said step of forming afiber preform includes the step of forming a preform having a generallytrapezoidal cross-section.
 5. The method of claim 1, wherein said stepof forming a fiber preform includes the step of forming a generallycylindrical fiber preform.
 6. The method of claim 1, wherein said stepof forming a fiber preform includes the step of winding at least onefilament about said central axis and providing additional windings atlocations corresponding to mesial and distal sides of the resultantbracket.
 7. The method according to claim 1, wherein said step ofclosing the mold assembly includes the step of advancing a projectiontoward the fiber preform to form an archwire slot and also the step ofadvancing at least one protrusion toward the fiber preform to form atleast one tiewing undercut.
 8. The method according to claim 1, whereinsaid step of forming a fiber preform includes the step of forming atubular fiber preform.
 9. The method of claim 1, including the step ofmixing together a quantity of Bis-GMA, TEGDMA and PEGDMA to make saidpolymeric material.
 10. The method of claim 1, wherein said step offorming a fiber preform having a central axis includes the step ofselecting reinforcing fiber structure having an index of refractionapproximately equal to the index of refraction of said polymericmaterial.
 11. The method of claim 1, including the step of mixingtogether a quantity of Bis-GMA, TEGDMA and PEGDMA to make said polymericmaterial.
 12. The method of claim 1, wherein said step of forming afiber preform having a central axis includes the step of selectingreinforcing fiber structure having an index of refraction approximatelyequal to the index of refraction of said polymeric material.